The present invention relates to an injection molding method for manufacturing a product having the external appearance free from defects such as sink marks or weld lines. The present invention is also directed to a mold assembly suitable for manufacturing such a product.
According to a method of injection molding a thermoplastic resin, the product which even has a complicated profile can be easily manufactured with high productivity. In this regard, the injection molding process has been adopted in various field, e.g. for manufacturing industrial parts or living necessaries.
A molten resin to be injection molded is forcibly poured through a gate into a molding cavity defined in a mold assembly. The poured resin passes through a predetermined channel to fill the cavity. The resin is diverged to a plurality of flows and then joined together during the fluidization in the cavity. When a plurality of the resin flows are incompletely joined together at the confluent point in the cavity, the incomplete joining causes the formation of weld lines. Especially in the case of manufacturing a product having a complicated configuration, a plurality of gates are opened to the molding cavity. Consequently, weld lines are formed on the surface of the product with high frequency.
The molten resin poured in the molding cavity is cooled and solidified in the state to fill the cavity. The resin reduces its volume due to phase transformation or thermal shrinkage during the cooling and solidification. The volumetric shrinkage is not uniformly promoted in the resin body, but is likely to concentrate at a certain part. The uneven shrinkage causes the formation of defects such as sink marks on the surface of an obtained product.
The resin especially at the thick wall part, where cooling and solidification would be delayed due to mass effect, is held in a highly fluid state, although the resin at the surroundings has been already solidified. Consequently, a stress originated in the shrinkage of the resin at the surroundings is accumulated in the resin at the thick wall part, so that sink marks are concentratedly formed on both the surfaces of the thick wall part.
Such a thick wall part is effective in an obtained product, e.g. as a reinforcing rib or a boss for fixture maldistributed on the periphery or interior of the product, or an outer or inner frame. However, surface defects such as sink marks or weld lines formed at the thick wall part or its neighborhood remarkably reduces the commercial value of the product. In general, the sink marks becomes deeper as the thickness of the rib, the larger diameter of the boss or the higher temperature of the molten resin.
For instance, when a molten resin 20 is poured into a cavity 13 between a stationary mold 11 and a movable mold 12 as shown in FIG. 1, the fountain flow 21 from which the molten resin 20 continuously gushes out is formed at the top end of the molten resin 20 passing into the cavity 13. The fountain flow 21, after being sent to the top end of the molten resin 20, changes along the direction toward the inner surfaces of the molds 11, 12 and then arrives at parts near the inner surfaces, as shown as a resin flow unit 22 in FIG. 1. The resin 20 is rapidly cooled and solidified by thermal diffusion through the molds 11, 12, to form a skin layer 23 in contact with the inner surface of each mold 11, 12. The cooling and solidification of the resin 20 is interpreted from the movement of the resin flow unit 22 as such that the resin 20 is stretched longer near the inner surface of each mold 11, 12 and formed to the skin layer 23 orientated due to the stretching.
When the molding cavity 13 in the mold assembly is filled with the molten resin 20, the molten resin 20 stops its fluidization. The interior of the molten resin 20 in the stationary state is cooled through the molds 11, 12, so that the skin layer 23 grows up to a thick solidified layer along the direction from the inner surface of each mold 11, 12 to the interior of the resin 20. At the same time when the solidified layer is formed, the molten resin 20 reduces its volume. Consequently, the interior of the molten resin 20 becomes at a negative pressure. The word "skin layer" in the following explanation means one involving said solidified layer.
In case where there is a wall part having thickness larger than that of the surroundings, the cooling and solidification of the molten resin 20 at the thick wall part is delayed in comparison with the cooling and solidification of the molten resin 20 at the surroundings. Due to said delay, the shrinkage strain of the molten resin 20 at the surroundings is accumulated in the thick wall part, so that a fairly large negative pressure is formed at the thick wall part. Said negative pressure affects on the skin layer 23, which is still viscous, at the thick wall part. Either of the skin layers 23, which is softer or less adhesive to the inner surface of the mold 11 or 12, is peeled off the inner surface. As a result, the surface of the solidified resin body is depressed. Hereby, sink marks are formed on the surface of an obtained resin product.
An injection molded product is ordinarily used with its flat surface faced to a user. A skin layer 23 for forming the flat surface shrinks along the inner surface of a mold during cooling and solidification, so that the skin layer 23 is easily peeled off the inner surface. On the other hand, a skin layer 23 for forming the back side of the product is hardly peeled off the inner surface of the mold, since the back side has a complicated profile due to the formation of ribs and/or bosses. Hereby, the skin layers for forming the front and back surfaces exhibit adhesiveness different from each other to the inner surfaces of the molds. Due to said difference in adhesiveness, the skin layer for forming the front surface is preferentially pulled inwards by the negative pressure in the interior of the molten resin. Thus, sink marks are formed on the front surface of the product.
There have been proposed various injection molding methods belonging to a short-shot process accompanied with high-pressure gas assist, for manufacturing a product having a very thick wall part. According to the short-shot process, a molten resin is injected into a molding cavity in an amount insufficient to completely fill the cavity, and a high-pressure gas is forcibly injected into the cavity so as to form a hollow portion in the molten resin. The high-pressure gas may be simultaneously injected into the cavity with the molten resin. The injected molten resin is dwelled by the application of the gaseous high pressure through the hollow resin passage. Hereby, the resin is cooled and solidified in a state pressed onto the inner surface of the mold.
In order to realize the effect of the high-pressure gas application, there shall be provided a thick wall part communicating with a gas passage. Herein, there are restrictions on the designing of a product, e.g. the size and position of the thick wall part, especially when the product to be injection molded has a complicated configuration. In addition, said thick wall part acts as a flow leader for the molten resin, so as to destroy the injection balance of the resin with respect to the whole body of the product. As a result, surface defects such as air traps and flow marks are easily formed on the surface of the obtained product.
Besides, the short-shot process necessarily uses the high-pressure gas. The use of the high-pressure gas requires dangerous operations, difficult handling and a special expensive control system. Due to these defects, the short-shot process has not been developed to broad extents.
By the way, there have been proposed other methods for inhibiting surface defects such as sink marks and weld lines with ease.
For intance, Japanese Patent Application Laid-Open 56-167410 teaches a local heating method, wherein the back side of a mold is partially heated at a position easy to form sink marks. A molten resin in contact with the heated mold part is delayed in cooling and solidification from the other part. When the back side at the position easy to form sink marks is partially heated at a temperature higher than that of the mold, sink marks are formed on the back side of the product but not on its front surface. Hereby, the obtained product does not substantially reduce its commercial value due to the sink marks. However, the local heating of the mold causes the elongation of an injection molding cycle and additionally requires heating means.
Japanese Patent Application Laid-Open 61-9126 teaches the provision of a porous nozzle at the part of the back side easy to form sink marks. Compressed air is introduced through the porous nozzle into the cavity so as to press the back side of aresin body. The resin is cooled and solidified in the state tightly pressed onto the inner surface of the mold by the application of the compressed air. Hereby, an obtained product has a smooth surface without the formation of sink marks. However, the method requires a specially designed mold in which the nozzle made of a porous material can be installed. In addition, the injection of the compressed air through the porous nozzle requires additional equipment and put restrictions on the structure of the mold.
A molten resin poured into a molding cavity is diverged to a plurality of flows and then joined in response to the configuration of said cavity. When a plurality of the resin flows are joined together, their skin layers are fused to each other. If the skin layers are fixed without sufficient fusion, a weld line is formed at the joining position. The weld line reduces the commercial value of a product, too.
There have been also proposed several methods for reducing weld lines. For instance, heating and cooling means are provided at the parts of a mold easy to form weld lines, as disclosed in Japanese Patent Application Laid-Open 51-22759. The fusion of the skin layers to each other can be promoted by the application of a shearing force to a part of a molten resin in the cavity, as disclosed in Japanese Patent Application Laid-Open 3-274127. Said fusion may be promoted by the transmission of ultrasonic waves to a molten resin in a fluid state, as disclosed in Japanese Patent Application Laid-Open 4-90309. These methods effectively inhibit the formation of weld lines, but requies additional devices or equipment. In this respect, there is a strong demand for the practical method which can easily inhibit the formation of weld lines.
Japanese Patent Application Laid-Open 4-211912 discloses the injection mold which can simultaneously inhibit the formation of weld lines and flow marks. Said mold has an inner surface coated with a heat insulating layer. The layer has thickness of 0.5-50 .mu.m, heat conductivity of 0.1 cal/cm.multidot.sec.multidot..degree. C. or less and an angle of contact with water not more than 30 degrees. The layer is made of a material such as metallic oxide, silicic oxide, silicic complex oxide and plasma-polymerized plastics.
When a molten resin injected into the molding cavity comes in contact with the heat insulating layer, the layer is instantaneously heated at a high temperature by a heat retained in the molten resin. The heated layer exhibits excellent wettability and conformability to the molten resin, so as to improve the transferring ability of the mold. Consequently, a product having the profile which accurately imitates the configuration of the molding cavity is obtained without the formation of weld lines or flow marks. However, the mold having the inner surface coated with the heat insulating layer does not have the effect to inhibit the formation of sink marks originated in the volumetric shrinkage of the molten resin during cooling and solidification.
In general, a molten resin injected into the molding cavity is rapidly cooled, just after it comes in contact with the inner surface of the mold. Hereby, a surfacial skin layer is formed on the surface part of the molten resin in contact with the inner surface of the mold. The cooling and solidification of the molten resin 20 continues while gradually thickening the surfacial skin layer inwards to the skin layer 23. Hereon, a negative pressure is formed in the interior of the molten resin 20 by the abrupt volumetric shrinkage which occurs at a glass-transition temperature. The negative pressure acts as the power to pull the skin layer 23 inwards shown by the arrows in FIG. 2, and becomes larger with the cooling and solidification of the molten resin 20. Once the negative pressure exceeds the self-supporting force of the skin layer 23, the skin layer 23 is peeled off the inner surface of the mold and pulled inwards. The resulting dent acts as a starting point for the formation of sink marks.
An object of the present invention is to adversely utilize the dent-forming phenomenon to obtain an injection molded product excellent in surface appearance.
Another object of the present invention is to provide a simplified method capable of inhibiting the formation of sink marks on the front surface of a product with ease, as well as the formation of weld lines caused by the joining of resin flows during the injection of a molten resin into a molding cavity.